1. Field of the Invention
The present invention relates to a mobile radio telecommunications system and, more particularly, to a mobile radio telecommunications system feasible for portable telephones. The present invention is also concerned with procedures for setting up connection for calls originating and terminating, switching a radio zone and a base station while a conversation is under way, and registering locations.
2. Description of the Related Art
Conventional mobile radio telecommunications systems include an automobile telephone and portable telephone system. It is a common practice with an automobile telephone and portable telephone system to arrange bidimensionally a plurality of base stations each covering a particular radio zone or service zone. The zones assigned to nearby base stations overlap each other in order to allow a conversation to be continued on a mobile station over the adjoining zones. This kind of system is referred to as a cellular radio telephone communication system.
In the conventional cellular radio telephone communication system, a mobile station accesses, for example, a landline telephone included in a wire-line telephone network by the following procedure. To begin with, the mobile station sends a calling signal to base stations adjoining it. In response, the base stations each sends a calling signal, including a received level thereof, to a radio channel control station. The radio channel control station selects one of the base stations whose received level is highest and determines that the mobile station exists in the zone assigned to that base station. Then, the control station sends the calling signal to a mobile telecommunications switching office. On receiving the calling signal, the switching office sets up connection between the base station of interest and the wire-line telephone network. To terminate a call at a mobile station, it is necessary to determine a zone currently covering the mobile station. This has customarily been done by calling a plurality of zones from the telephone network side by broadcasting in response to every terminating call, detecting a response from a mobile station of interest, i.e., a base station having the highest received level in order to locate the mobile station, and then terminating the call at the mobile station.
Assume that a mobile station is transferred from one zone to another zone while a conversation is held thereon. Then, to switch the zone, the base station whose zone currently covers the mobile station constantly monitors the level at which it receives a radio wave from the mobile station. When the received level of the base station falls below a predetermined value, the base station determines that the mobile station has been transferred to another zone, and then sends a level decay signal to the radio channel control station. In response, the control station sends a level monitor request to base stations around the base station generated the level decay signal. As such base stations send their received levels to the control station, the control station compare them and selects one of the base stations whose received level is highest. Subsequently, the control station sets up a path between the switching office and the base station selected, and then commands the previous base station to disconnect the channel.
A mobile station memorizes the current base station code at all times. Assume that the mobile station has traversed a given zone or location registration area, which is a location identification unit. Then, the mobile station compares a base station code with the memorized base station code and determines that it has traversed the zone on the basis of a change of base station code. Then, the mobile station sends a location registration signal, including a mobile station identification number, to a base station toward which it is moving. The base station at the destination transfers the location registration signal to the switching office via the control station. The switching office selects a home memory station associated with the mobile station identification number and then transfers the location registration signal to the home memory station. In response, the home memory station updates the location information relating to the mobile station. Further, the switching office received the location registration signal returns a confirmation signal to the mobile station to cause it to update the base station code.
The conventional cellular radio telephone communication system described above has some problems left unsolved, as follows. It has been customary to assign a particular frequency, or channel, to each of a plurality of nearby zones in order to avoid wave interference. To promote effective use of limited frequency resources, each zone should desirably be implemented as a microzone, as in a cordless telephone system, and should use the same frequency repeatedly. However, microzones increase the number of times that the frequency is switched due to the movement over adjoining zones, resulting in extra loads on both of base stations and mobile stations.
Another problem is that wave propagation troubles, e.g., fading and shadowing obstruct the radio communication between a base station and a mobile station. Generally, in a cellular radio telecommunications system, a substantially circular zone is assigned to each base station and has a diameter of several kilometers to several ten kilometers. This prevents cordless telephones or similar small-power portable telephones from being applied to such a zone configuration, since wave propagation distance available therewith is limited.
Moreover, to accommodate a greater number of subscribers, it is a common practice to increase the number of zones or base stations while reducing the size of each zone. However, the number of sites available for base stations is not great enough to further increase the number of subscribers.